The blade pitch system in wind turbines consists of a bearing on which the blade is attached. This bearing is joined by one part, i.e. the fixed or stationary part, to the hub, whilst the moving part is joined on one side to the blade and on the other side, the part located inside the hub, to a driving plate. The said blade, in turn, is connected, by means of a hydraulic or electric actuator, to the hub which makes the blade to rotate. In the case of a hydraulic actuator, the said actuator moves the driving plate through a ball joint system.
The driving plate has two functions: on the one hand it stiffens the hub, that is, it has a structural function and on the other hand it acts as a link in the blade pitch drivetrain system.
The bearing has the dimensions of the blade root and is generally a bearing in which the contacting surfaces with the blade and the driving plate are perpendicular to the axis of rotation of the blade pitch bearing so that the longitudinal direction of the blade is coincident with the axis of the bearing, wherein said driving plate in turn has its bearing contact surface perpendicular to the longitudinal direction of the blade.
However, it has been proven beneficial, due to the loads born by the wind turbine, that the axis of rotation of the blade pitch system and the longitudinal direction of the blade are not coincident, but rather have an angle between them such that the tip moves away from the tower so that when the loads are at a maximum, at the corner zone of the power curve, the blade, as the blade flexes, the center of gravity gets closer to the bearing axis of rotation The said loads on the blade and on the wind turbine are thus considerably reduced. This angle is called the preconing angle.
This geometric aspect is very important as it permits a lightening of the blade designs and of the other components of the wind turbine. To achieve the preconing angle there are two possible alternatives: arranging the surfaces of the moving bearing ring so that they are not parallel to their axis, by means of machining, as disclosed in European patent application EP2336553A1, or by machining the blade root to the required angle.
Machining the root of the blade to the required angle turns out to be impractical given the precision required for the said angle, so the usual choice is to machine the bearing, which increases the amount of material required for its manufacture, because the amount of raw material required is greater than that of the final bearing, where the difference is lost in the machining process.
It is also advantageous to have just one platform, i.e., one with the same wind turbine structural elements, namely the tower and nacelle elements, with the same dimensioning, such that the said platform may be valid for use with different sizes of blades and for different types of installation, thereby optimizing the cost of manufacture.
However, loads may vary from one wind turbine to another depending on the blade used, making it difficult to maintain the same platform. One way of resolving this is to customize the preconing angle for each rotor. It is therefore of great interest to employ methods and techniques that allow the distance between the blade and the tower to be easily adapted from one platform to another.
Wind turbine systems with blade pitch actuation systems known in the prior art comprise a bearing, a driving plate, a shaft or ball joint system and a hydraulic linear actuator, wherein the blade is arranged in contact with a first surface of the moving bearing ring, and the driving plate, also called pitch plate, is placed in contact with a second flank of the moving bearing ring facing the interior of the hub. The driving plate is connected to the actuator which is joined to the hub through a ball joint system. The fixed bearing ring is attached to the hub of the nacelle. The introduction of the ball joint system is necessary because the angle formed between the longitudinal axis of the blade and the rotational axis of the fixed bearing ring makes it necessary for the actuator to maneuver, rotating with a conical rather than cylindrical orbit. If the orbit was perfectly cylindrical the said ball joint system would not be necessary.
Furthermore, the driving plate and the shaft, or ball joint system, are manufactured separately and assembled subsequently, generally using bolts to connect them.
In European patent application EP2336553A1 the blade pitch system is given a preconing angle by machining the bearing so that the face that is in contact with the blade is not perpendicular to the axis of rotation of the blade pitch system.
However, since the wind turbines have very large blades, even exceeding 40 meters, a small error either in the machining or in the assembly results in a large error in the position of the blade tip. It is the distance between blade tip and the tower which determines in many cases, the dimensions of the blade structure; thus the machining of the bearing surface must be very precise and the tolerance must be very narrow. This greatly increases the cost of the bearing which must be machined and assembled with the rest of the components in a way so as to ensure the said angle, moreover the amount of waste material is greatly increased relative to a straight bearing.
All these problems are solved with the wind turbine blade pitch system of the present invention.